Problem: 9 markers cost $12.78. Which equation would help determine the cost of 4 markers?
Solution: There are several equations that could help determine the cost, each with a slightly different approach. Let $x$ represent the unknown cost of 4 markers. Since 4 markers cost $x$ , we have the following proportion: $\dfrac{4}{x}$ We can write the fact that 9 markers cost $12.78 as a proportion: $\dfrac{9}{\$12.78}$ The cost changes along with the number of markers purchased, and so the two proportions are equivalent. $\dfrac{4}{x} = \dfrac{9}{\$12.78}$